The present invention relates to a speed control apparatus for DC motor which reduces to a minimum undesirable change in revolution speed of the motor due to change of source voltage and which is suitable for constructing on a semiconductor integrated circuit.
Electrically controlling the speed of a DC motor utilizing counter electromotive force induced in the armature winding and proportional to the revolution speed of the DC motor to be controlled, is known. FIG. 1 shows a practical example of the apparatus for such method.
In the conventional apparatus of FIG. 1, an equivalent internal resistance R.sub.a of a DC motor 1 to be controlled, a resistor 2 of resistance R.sub.1 joined in series with the DC motor and resistors 3 and 4 of resistances R.sub.2 and R.sub.3, respectively, joined in series form a bridge circuit. When equilibrium condition of the bridge (that is, R.sub.1 .multidot.R.sub.2 =R.sub.3 .multidot.R.sub.a) is fulfilled, the voltage across the detection output terminals "a" and "b" corresponds directly to the revolution (rotational) speed of the DC motor 1 and is not dependent on load torque or armature current of the DC motor 1. Therefore, by obtaining a difference voltage between a voltage which is proportional to counter electromotive force of the DC motor 1 and a reference voltage which is selected for defining a desired revolution speed, by amplifying the difference voltage by a differential amplifier and feeding an amplified output to a driver transistor 5, by driving a current controlling transistor 7 thereby, and by increasing or decreasing the current to the DC motor 1 when the revolution speed is lower or higher than the preset value, the revolution speed of the DC motor 1 can be controlled almost to the preset value. To produce the preset reference voltage, a current is fed through a resistor 9 to the diode 8. Forward voltage drop of the diode 8, which is substantially constant, is divided by a dividing network consisting of resistors 10 and 11, and the reference voltage is given across the resistor 11. The divided point of the dividing network 10+11 is connected to the base of a transistor 12 of a differential amplifier, and the terminal " b" (that is, the junction point of the resistors 3 and 4) is connected to the base of the other transistor 13 of the differential amplifier. The resistor 11 is connected between the terminal "a" and the base of the transistor 12, which is an input terminal "c" of the differential amplifier. A current is fed from the positive terminal of the DC power source 6 to the differential amplifier 12+13 through a resistor 14. A capacitor 15 prevents undesirable oscillation.
In the conventional speed control apparatus of FIG. 1, the controlled speed is influenced by changes of voltage of the power source 6 as follows:
The reference voltage which defines a selected revolution speed is given as a voltage across the resistor 11 which is produced by supplying a current from the power source 6 in the series circuit of a resistor 9, the diode 8 and the resistor 2, and dividing the forward voltage drop of the diode 8, as a constant voltage device, by the dividing network 10+11. Therefore, when the voltage of the power source 6 increases, the current in the series circuit consisting of the resistor 9, the diode 8 and the resistor 2 increases, and the forward voltage drop across the diode 8 increases, thereby increasing the voltage across the resistor 11. As the voltage across the resistor 11 increases, the revolution speed of the DC motor 1 increases. On the other hand, when the voltage of the power source 6 decreases, the current in the diode 8 decreases, thereby decreasing the forward voltage across the resistor 11. As the voltage across the resistor 11 decreases, the revolution speed of the DC motor 1 decreases. As mentioned, a problem in the conventional speed control apparatus of FIG. 1 is that as the power source voltage changes, the revolution speed of DC motor fluctuates considerably.